@sack-o-rine. Holy crap. It was right there the whole time. The girl in Senpai's manga from chapter 1 was based off the art club president, similar to how Siegfried was Senpai's self-insert (according to Nagatoro). Similar hairstyle, same face, they even have the same personality/dedicated to their work, with of course some "artistic changes" from Senpai's part. It makes sense that Senpai would give his self-insert a love interest based on a girl he liked in real life. Plus, with her leaving to do cram study, Senpai, during chapter 13, turned that into Siegfried's companion leaving for her homeland. The girl from Senpai's manga wasn't some idealized fantasy, she was the girl that got away.